Page 328 - Drilling Technology in Nontechnical Language

Chapter 13 – DRILLING PROBLEMS AND SOLUTIONS 319

the filter cake thickness (and therefore the contact area) must be reduced.

Casing is a particular problem, due to the large-diameter, smooth pipe
giving lots of good contact area with the formation.

To prevent differentially stuck pipe, the mud should be tailored so

that it forms a thin, tough and nonsticky filter cake. Overbalance should
be minimized, commensurate with maintaining primary well control.
Stabilizers can be run to hold the large-diameter drill collars off the
wall. Drill collars can be used that have spiral grooves cut into them,
which reduces the contact area. Finally, whenever the BHA is opposite a
permeable formation, the time it spends static (neither reciprocating nor
rotating) should be minimized.
Sometimes, the drillstring or casing will get stuck by another
mechanism elsewhere in the hole and then will become differentially stuck
if the conditions exist.
Once pipe becomes differentially stuck, chemicals can be pumped
down the drillstring and over the sticking formation to dehydrate and

shrink the filter cake. Reducing overbalance is more tricky, first because

of the well control implications and second because reducing mud
hydrostatic significantly can have a highly destabilizing effect on some

shales. The other action that can be taken is to pull and jar on the pipe. This
is described next.

Jars and jarring

Steel as a material is quite elastic. Elasticity is a property of a material
whereby a force applied to the object causes it to deform, and when the
force is removed, the object reverts to its original dimensions. Springs are
made of steel precisely because it is an elastic material. If more than a
certain force is applied, the material stretches permanently; this force is
called the elastic limit. If even more force is applied, the material will
break; this force is called the ultimate tensile strength. Chapter 5 discussed
how these limits affect the design of the drillstring.
When the drillstring is held in tension, it has stretched to a certain
extent. How much it has stretched is measured by the strain. Strain is
simply the amount that the pipe has stretched divided by the original
length. So if a 10,000 ft pipe stretches by 5 ft, the strain will equal 0.0005.

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